The present invention relates to an imaging system using coherent light radiation to expose a layered member in an image configuration and more particularly, to a method for directing the light radiation into the imaging member so as to reduce optical interference occurring within said photosensitive member which results in a plywooding type of defect in output prints.
There are numerous applications in the electrophotographic art wherein a coherent beam of radiation, typically from a helium-neon or diode laser is modulated by an input image data signal. The modulated beam is directed (scanned) across the surface of a photosensitive medium. The medium can be, for example, a photoreceptor drum or belt in a xerographic printer, a photosensor CCD array, or a photosensitive film. Certain classes of photosensitive medium which can be characterized as "layered photoreceptors" have at least a partially transparent photosensitive layer overlying a conductive ground plane. A problem inherent in using these layered photoreceptors, depending upon the physical characteristics, is an interference effectively created by two dominant reflections of the incident coherent light on the surface of the photoreceptor; e.g., a first reflection from the top surface and a second reflection from the bottom surface of the relatively opaque conductive ground plane. This condition is shown in FIG. 1: a coherent beam is incident on a layered photoreceptor 6 comprising a charge transport layer 7, charge generator layer 8, and a ground plane 9. The interference effects can be explained by following two typical rays of the incident illumination. The two dominant reflections of a typical ray 1, are from the top surface of layer 7, ray A, and from the top surface of ground plane 9, ray C. The transmitted portion of ray C, ray E, combines with the reflected portion of ray 2, ray F, to form ray 3. Depending on the optical path difference as determined by the thickness and index of refraction of layer 7, the interference of rays F and E can be constructive or destructive when they combine to form ray 3. The transmitted portion of ray 2, ray G, combines with reflected portion of ray C, ray D, and the interference of these two rays determines the light energy delivered to the generator layer 8. When the thickness is such that rays E and F undergo constructive interference, more light is reflected from the surface than average, and there will be destructive interference between rays D and G, delivering less light to generator layer 8 than the average illumination. When the transport layer 7 thickness is such that reflection is a minimum, the transmission into layer 8 will be a maximum. The thickness of practical transport layers varies by several wavelengths of light so that all possible interference conditions exist within a square inch of surface. This spatial variation in transmission of the top transparent layer 7 is equivalent to a spatial exposure variation of generator layer 8. This spatial exposure variation present in the image formed on the photoreceptor becomes manifest in the output copy derived from the exposed photoreceptor. FIG. 2 shows the areas of spatial exposures variation (at 25.times.) within a photoreceptor of the type shown in FIG. 1 when illuminated by a helium-neon laser with an output wavelength of 633 nm. The pattern of light and dark interference fringes look like the grains on a sheet of plywood. Hence the term "plywood effect" is generically applied to this problem.
There are several methods for compensating for the plywood effect known in the prior art. These methods fall into two general techniques; to either change the structure of the imaging member to reduce the second dominant reflection from the imaging member ground plane or to reduce or eliminate the first reflection from the top surface. Some prior art references directed toward reducing the second dominant reflections are: U.S. Pat. No. 4,618,552 and co-pending application U.S. Ser. No. 07/546,990, assigned to the same assignee as for the present application, both of which describe methods for roughening the surface of the ground plane of the imaging member to create a diffuse reflection of the light reflected therefrom.
The present invention is directed toward reducing the first dominant reflection, e.g. the reflection from the top surface of the imaging member. This reflection can be reduced by diffusing the light from the top surface by roughening the surface of the imaging member as disclosed in U.S. Ser. No. 07/546,214 assigned to the same assignee as for the present application. The incident radiation can also be modified by a technique described in application Ser. No. 07/546,214, filed on Jun. 24, 1990, and assigned to the same assignee as the present invention, which discloses a method for merging scanned beams from 2 or more diodes at a photoreceptor surface. The beams are at different wavelengths producing an exposure variation pattern at the surface which compensates for the plywood exposure.
These prior art techniques for reducing the first dominant reflection may not be suitable for all systems; many applications may not tolerate a roughened top surface for the imaging member, and the merged beam technique requires two separate laser sources and may be prohibitively expensive. The present invention is directed toward a method for reducing plywood by practically eliminating the top surface reflection that combines with the inner reflection from ground plane E to cause the interference effects generating our exposure variations. There will still be some reflected light but this light will not vary with the thickness of transport layer 7, as there is no longer any interference, so there is no spatial variation in the resulting xerographic image. This elimination is accomplished according to a first aspect of the present invention by directing light from a laser source which is polarized in a plane parallel to the plane of incidence onto the photoreceptive surface at the Brewster angle, appropriate to the index of refraction of the media at both sides of the interface. It has been found that polarized light incident at this angle eliminates the reflected light and that angles close to the Brewster angle have minimal reflection that provides a significant reduction in defect level over that observed in prior implementations. This latitude with respect to incident angle can be incorporated into the design of scanning systems that must vary the angle in order to write information. More particularly, the invention is directed toward a method for increasing absorption (over the level of absorption observed with incoherent light) of incident coherent light being scanned across the surface of a photosensitive imaging member moving in a process direction comprising the steps of:
generating beams of high intensity modulated coherent light polarized in the plane parallel to the plane of the incident light,
optically directing said beam onto the surface of said imaging member at an angle whose value is defined by the relationship EQU (tangent .THETA.=(n'/n)
where n is the index of refraction of the first medium through which the beam travels prior to being incident at the surface of the imaging member and n' is the index of refraction of a second medium adjacent the first medium, n' being greater than n,
whereas the incident light is almost totally transmitted by the top layer (due to the elimination of the reflection) resulting in a maximum delivery of illumination to the inner light generator absorbing layer 8.